I've used this tool a bit, it's the good kind of voodoo, but it desperately needs modernization, it looks like it was forged in the '90s and was never updated.
Of course, you can change the defaults, but creating the 'look' you want textually without an interactive creator is non-trivial. Given that the main strength of graphviz is that I can write it in a stream of consciousness (like markdown), or generate it automatically using some kind of script from another data source, this is somewhat of a blocker in its usefulness.
And, of course, the customisation syntax is fairly clunky and not easily readable. I don't think I've seen a DSL that does colouring well in this regard, but graphviz goes a step further by not being consistent with CSS or other markups.
It is good in terms of feature-completeness, but as mentioned in the TC, there are areas where it could be polished up a bit to make it feel more modern.
Edit: found it https://github.com/vidarh/diagram-tools/tree/master
And then the graphics need to look more D3 than UML.